1. Field of Endeavor
The present invention relates to the field of climate protection technology, and in particular to reduced complexity and increased efficiency of carbon capture in combined cycle power plants, in which waste heat from a gas turbine engine is used to raise steam for a steam turbine.
2. Brief Description of the Related Art
Devices for production and separation of carbon dioxide (CO2) from gas streams, using cryogenic separation units in the form of vortex nozzles (also called cyclonic separators), are known from prior published patent applications WO-A2-03/029739 and US-A1-2003/0145724. To operate efficiently at ambient temperatures, such vortex nozzles require to be fed by cooled gas streams pressurized to at least 2 or 3 bar at the inlets to the vortex nozzles. Within the vortex nozzles, the gases expand rapidly to much lower pressures, rapidly cooling the gas. A prior International patent application PCT/EP2007/057434, filed 7 Aug. 2006, uses vortex nozzles in an improved process for separating CO2 from a gas flow, such as the exhaust from a gas turbine engine burning a fossil fuel. In general terms, the process includes: compressing the gas flow to a pressure of about 2-3 bar, cooling it down to about −40° C. to −50° C., supersonically expanding it through vortex nozzles so that solid CO2 centrifugally precipitates from the rest of the gas flow, and discharging the CO2 from the outer wall of the vortex nozzle for further treatment, such as preparation for sequestration.
FIG. 1 diagrammatically illustrates a known type of sequential arrangement of plant components for a CO2 capture process specifically adapted for use in conjunction with a combined cycle power plant. A gas turbine engine 10 compresses intake air 11 in a compressor 12, burns fuel in the compressed air 13 in a combustor 14, and obtains work from the combustion gases 15 in a turbine 16, which drives the compressor 12 via a common shaft 17. The major part of the mechanical power developed in turbine 16 is used to drive the electrical generator G. After expansion through the turbine 16, the exhaust gases 18 are typically at about atmospheric pressure and contain about 3-4 volume % CO2. The hot exhaust or flue gases 18 exit from the exhaust duct of turbine 16 and are passed through a heat recovery steam generator (HRSG) 19 which raises steam 20 for expansion through a steam turbine 21 to generate further power from a generator G, driven through a shaft on which steam turbine 21 is mounted. After exit from the steam turbine 21, the wet steam 22 is passed through a condenser 23, and the condensed water 24 is then recycled to the HRSG 19 by pump P. After giving up much of their heat in the HRSG 19, the flue gases 25 remain at about atmospheric pressure, but have been reduced to a temperature of about 80-120° C. Gases 25 are then cooled down to approximately ambient temperature (typically the temperature of available cooling water+10 K) in a heat exchanger 26. The cooling water used in the heat exchanger 26 may, for example, be re-cooled in one or more cooling towers, or environmental water from a river, lake or sea, could be used to cool the flue gases 25.
As previously mentioned, carbon dioxide separation in a vortex nozzle requires the gases to be pressurized in the range 2 to 3 bars, at least. The cooled flue gas 27 is therefore compressed in a gas compressor 28, driven by a motor M1, which may be powered by electricity generated by the gas turbine 10 and the steam turbine 21. Alternatively, the gas compressor 28 maybe directly coupled to the shafts of either the gas or the steam turbine. The compressed flue gases 29 must then be cooled down to a temperature of −40 to −50° C. before the CO2 can be cryogenically separated in a set of vortex nozzles 38 whose inlets are arranged to receive flue gases in parallel with each other (only one nozzle is shown). This is achieved by a flue gas cooling system operating in a three-stage process. In the first cooling stage, the compressed flue gas is cooled back down towards ambient temperature (again, typically the temperature of available cooling water+10 K) in a suitable heat exchange arrangement 30. The second and third stages comprise active cooling cycles or other refrigeration apparatus. In the present example heat pumps are used, these being generally indicated by numerals 33 and 36. As known, each heat pump 33/36 includes an evaporator 331/361, a compressor 332/362 driven by a motor M2, a condenser 333/363, and a metering valve 334/364. Hence, in the second cooling stage, flue gas 31 from the first cooling stage passes through the evaporator 331 of active cooling cycle 33. Flue gas 34 leaves evaporator 331 at a temperature which is 2 to 5 K above the freezing point of water, the evaporator being equipped with a suitable known device for separating condensed water from the flue gas. In the third cooling stage, flue gas 34 from the second stage is further cooled down to the required temperature of −40° C. to −50° C. by an evaporator 361 of the second active cooling cycle 36. Evaporator 361 must be equipped with a suitable device for removing ice deposited on the heat exchanger surfaces during cooling of the flue gas. Finally, compressed cooled flue gas 37 enters the vortex nozzles 38, where it is cooled by expansion and centrifugally separated into a CO2 stream 39 and a residual flue gas stream 43. The CO2 stream 39 is cleaned, compressed by gas compressor 40 and fed into a pipeline 41 for storage, while the residual flue gas 44 is discharged into the atmosphere through a flue, stack (S), or the like, after undergoing further environmental cleaning procedures, if necessary.
To increase system efficiency, the condenser 363 for refrigerant in the second active cooling cycle is placed downstream of the vortex nozzles 38 so that the cold, CO2-depleted flue gas 43 exhausted from the vortex nozzle 38 can be used as a heat sink.
The CO2 capture/separation process will not be further described here because it is adequately described in the above-mentioned prior patent application and other prior art.
Without the provision of CO2 capture equipment in FIG. 1, the flue gas would simply be ejected to atmosphere after leaving the HRSG 19. However, to obtain the cool pressurized flue gas required for feeding into the vortex nozzle 38, a compressor 28 and the cooling units 26, 30, 331, and 361 are required, resulting in plant complexity and costs in construction, running and maintenance.